This invention relates to medical devices, and more particularly to methods of manufacturing medical devices using fine grained metals and alloys.
A focus of recent development work in the treatment of heart disease has been directed to endoprosthetic devices referred to as stents. Stents are generally tubular shaped devices which function to maintain patency of a segment of a blood vessel or other body lumen such as a coronary artery. They also are suitable for use to support and hold back a dissected arterial lining that can occlude the fluid passageway. At present, there are numerous commercial stents being marketed throughout the world. Intraluminal stents implanted via percutaneous methods have become a standard adjunct to balloon angioplasty in the treatment of atherosclerotic disease. Stents prevent acute vessel recoil and improve the long term outcome by controlling negative remodeling and pinning vessel dissections. Amongst their many properties, stents must have adequate mechanical strength, flexibility, minimal recoil, and occupy the least amount of arterial surface area possible while not having large regions of unsupported area.
One method and system developed for delivering stents to desired locations within the patient's body lumen involves crimping a stent about an expandable member, such as a balloon on the distal end of a catheter, advancing the catheter through the patient's vascular system until the stent is in the desired location within a blood vessel, and then inflating the expandable member on the catheter to expand the stent within the blood vessel. The expandable member is then deflated and the catheter withdrawn, leaving the expanded stent within the blood vessel, holding open the passageway thereof.
Stents are typically formed from biocompatible metals and alloys, such as stainless steel, nickel-titanium, platinum-iridium alloys, cobalt-chromium alloys and tantalum. Such stents provide sufficient hoop strength to perform the scaffolding function. Furthermore, stents should have minimal wall thicknesses in order to minimize blood flow blockage. However, stents can sometimes cause complications, including thrombosis and neointimal hyperplasia, such as by inducement of smooth muscle cell proliferation at the site of implantation of the stent. Starting stock for manufacturing stents is frequently in the form of stainless steel tubing.
The structural properties of the material used for implantable medical devices can improve with a decrease in the grain size of the substrate material. It has been observed that stents cut from fully annealed 316L stainless steel tubing having less than seven grains across a strut thickness can display micro cracks in the high strain regions of the stent. Such cracks are suggestive of heavy slip band formation, with subsequent decohesion along the slip planes. Reduction of the grain size in the substrate material, such as stainless steel, will reduce or eliminate the occurrence of such cracks and/or heavy slip band formation in the finished medical device.
The grain size of a finished stainless steel or similar metal tube depends on numerous factors, including the length of time the material is heated above a temperature that allows significant grain growth. For a metallic tube, if the grain size is larger than desired, the tube may be swaged to introduce heavy dislocation densities, then heat treated to recrystallize the material into finer grains. Alternatively, different material forms may be taken through a drawing or other working and heat treat processes to recrystallize the tubing. The type and amount of working allowed depends on the material, e.g., ceramics may require a high temperature working step while metals and composites may be workable at room temperature. Grain-size strengthening is where there is an increase in strength of a material due to a decrease in the grain size. The larger grain-boundary area more effectively blocks dislocation movement. The outer diameter of the tube usually requires a machining step of some sort to smooth the surface after the swaging process, and the same may be true before the tubing can be properly drawn.
Commercially available 316L stainless steel tubing contains average grain sizes ranging from approximately 0.0025 inch (sixty four microns), ASTM grain size 5 to around 0.00088 inch (twenty two microns), ASTM grain size 8. These grain sizes result in anywhere from two to five grains across the tube thickness, and the stent subsequently manufactured from the tubing, depending on the tube and stent strut thicknesses. Part of the limitation in achieving a finer grain size in this material arises from the number of draws and anneals the tubing must go through to achieve its final size. The potential for reducing the grain size exists by reducing the required number of heat-processing steps by reducing the starting size of the raw product that is then processed down into the tubing.
Lowering the grain size and increasing the number of grains across the strut thickness of a stent allows the grains within the stent to act more as a continuum and less as a step function. The ideal result of processing the material to a smaller grain size would result in an average grain size of between approximately one and ten microns, with a subsequent average number of grains across the strut thickness about seven or greater. Likewise, other medical devices will benefit from a reduction in grain size such as guide wires, ring markers, defibrillator lead tips, delivery system devices such as catheters, and the like.
What has been needed, and heretofore unavailable, in the art of medical device design is fine grained metals and alloys that have uniform and predictable properties and that contain grain sizes on the order of one to ten microns. The present invention satisfies these and other needs.